1. Technical Field of the Invention
The present invention relates in general to optical rotation sensors, and in particular to optical rotation sensors with higher sensitivity and reduced lock-in.
2. Description of Related Art
An Optical Rotation Sensor (ORS) is an instrument used to measure the angular rotation rate of a certain platform. The two main well-known configurations of ORS's are the Ring Laser Gyroscope (RLG) and the Fiber Optical Gyroscope (FOG).
An RLG typically includes a laser system designed and fabricated to work in a ring configuration. The most familiar form of an RLG is built using a helium-neon (HN) ring laser with a diameter in the range of 30 cm. The operating principle of an RLG is based on the beating between two counter propagating beams of light in the ring laser cavity.
An FOG typically includes a wide-spectrum source, such as an SLED, connected to a fiber coil through a 50/50 directional coupler. The fiber coil is turned in a ring shape to form a ring mirror with a variable reflection coefficient according to the rotation rate.
At stationary state, the two beams in an ORS travel the same distance around the cavity, and thus have the same phase shift. When the system is rotated with a certain specific angular rotation rate, one beam experiences a larger distance around the cavity than the other due to the Sagnac effect, and as a result, the two beams are generated at two different wavelengths (in the case of an RLG) or the reflection coefficient changes according to the rotation rate (in the case of an FOG). The optical path difference between the two beams is directly proportional to the rotation rate of the cavity or the minor, and similarly, the optical frequency difference or the minor reflection coefficient. Such a difference can be detected as a beating frequency between the two waves propagating in the ring laser in the clockwise (CW) and counter clockwise (CCW) directions in RLGs or as an amplitude modulation of the intensity (i.e., change in DC values from the detector) in FOGs.
One of the main problems in RLG systems is coupling and lock-in between the two propagating beams at low rotation rates. Due to the nature of the optical cavity, a scattering mechanism takes place at the reflector interfaces. Such scattering causes energy to be coupled from the CW beam to the CCW beam and vice-versa. This coupling can cause the two beams to be pulled to the same frequency in a phenomenon called mode lock-in, which seriously limits the sensitivity of RLG devices. Another challenge in RLG instruments is determining the direction of rotation of the platform. Therefore, there is a need for an improved ORS with reduced lock-in.